A mechanical system in normal operational condition comprises a certain amount of frictional force due to friction that resists movement. If the magnitudes of the frictional forces in the system can be determined by measuring or mathematically, this information can be utilized as an indicator of the operational condition of the system.
An elevator system contains numerous components that are exposed to chafing and wear. The motion of the elevator car causes wear of components, including e.g. the elevator ropes and the guide rails of the elevator car. One of such components is the elevator door, which moves automatically on a horizontal rail. It is acted on by forces applied to it from different directions, and both its upper and lower edges are in contact with rails keeping the door movement on its track. There is also a frictional force opposing the motion of the automatic door. The operation of the door may be disturbed when a sufficient amount of dirt is accumulated on the door rail on the threshold of the elevator car. Due to this physical obstruction, the force opposing the motion of the door may grow to a magnitude such that finally the door control system is no longer able to open or close the door.
The magnitude of the frictional force can not be measured directly. It is not possible to mount a separate “friction meter” on the door. The magnitude of the friction resisting the movement of the door has to be measured indirectly. It is possible to create a model of the system to be examined, in this case the elevator door, to study the forces applied to the door. One of the forces appearing in the model is the frictional force opposing the motion. Using the model, it is possible to calculate desired parameters when the magnitudes of the forces opening and closing the door are known and the acceleration or velocity of the door is measured. In this way, unknown parameters, such as frictional force, can be solved. Thus, the matter at hand is a problem of optimization and estimation of parameters.
For example, in an elevator system the door assembly consists of a car door moving with the car and the landing doors on different floors. A modern automatic elevator door is opened and closed by means of a direct-current motor. The torque produced by the direct-current motor is directly proportional to the motor current. The energy of the motor is transmitted to the door e.g. via a toothed belt and the door moves on rollers. For reasons of safety, the landing door alone is closed without a motor by means of a closing device. The closing force of the closing device may be produced by a closing weight or a helical spring. The motor current and the corresponding torque are measured either from the door control card or directly from a motor current conductor. It is also possible to monitor a so-called tacho pulse signal of the motor. The tacho signal is a square wave whose frequency depends on the motor speed and therefore the door speed.
The problem with prior-art solutions is that the frictional force acting on the elevator door can not be measured directly. This necessitates the use of an indirect method of estimating the magnitude of the frictional force. The magnitude of the frictional force is needed for an estimation of the time to failure of the door or for predicting a future time by which the operational condition of the door will decline to a level consistent with a given criterion.